1. Field of the Invention
This invention relates to the switch-over of optical links and consequent rerouting of message packets in routers that terminate the optical links. The invention particularly relates to optical networks using the SONET protocol and particularly those using SONET Automatic Protection Switching (APS 1+1 and APS 1:1) arrangements to switch data traffic from a failed link to a redundant standby link.
2. Background Information
SONET is well known and described in a number of publications, e.g., Synchronous Optical Network (SONET), published by the International Engineering Consortium. Also well known is the use of APS described, for example, in a paper titled Cisco BPX 8600 Series Switches SONET Automatic Protection Switching (APS) and articles cited therein. As described in the latter publication, a failure of a working link in a SONET system can be rectified within the system by switching traffic to one or more backup links, generally within a fraction of a second. However, the network routers that terminate the SONET paths must also switch their data paths to the SONET backup path and this process may take substantially longer. For example, in a network using Open Shortest Path First (OSPF) path determination, a change in the path structure for packets passing through a router, requires the router to rebuild the link state data base from which its routing tables are derived. This operation takes a significant length of time: new adjacencies of neighboring routers must be established and those routers must exchange the link-state data bases from which they derive their routing tables. OSPF is described, e.g. in Internet Engineering Task Force Request for Comments 2328.
The rather short interruptions within the optical system due to switching of optical paths can be accommodated by buffering at the receiving end of the data string. On the other hand, the much longer delay resulting from the need to rebuild the link-state data bases in the routers causes an unacceptable delay when the data stream comprises audio or video packets.